I have a Novara that has a hard plastic covered with what is now slime. I just tossed the bars aside as they didn't meet my needs. But, perhaps I'll play around with them later, although the "slime" is bad.

I think the problem with several of the methods suggested above is that they require space below the grips, which the hard plastic prevents. Shoving in a screwdriver would be more likely to crack the plastic.

The alcohol might help.